Regarding a light-water reactor presently used in our country, there are two kinds of a pressurized water reactor (PWR) and a boiling water reactor (BWR).
Regarding a containment vessel of a pressurized water reactor, one with a cylindrical configuration is used so as to grant a withstand pressure capability. Accordingly, its upper structure is formed in a semispherical shape (dome shape) so as to cover the cylindrical portion. (JP-A 2008-157744 (KOKAI), for example).
On the other hand, as it is not necessary to grant a withstand pressure capability to a building used for a boiling water reactor, a building body is made as a rectangular shape in plane cross section by the reason for easiness and so on in constructing the building.
For this reason, a roof of a nuclear power plant building used for a boiling water reactor is generally formed in a planer shape or in a convex three dimensional shape so as to cover over the rectangular building body portion (JP-A 2001-42076, JP-A H2-281192 (KOKAI), for example).
This roof is of a steel truss structure so as to strive for weight saving, and its material is composed of reinforced concrete or iron plate.
The above-described nuclear power plant building is sited at a spot where it is not necessary to stochastically consider an incoming flying object such as a falling of a large-size commercial airplane or a fighter plane.
Thus, for the conventional nuclear power plant building, a collision with an airplane is not considered in its design condition.
However, in case that a building is sited at a spot where it is necessary to consider stochastically an incoming flying object, or in case that it is necessary to consider a falling of an airplane in its design condition assuming terrorism and so on, there is a possibility not to guarantee a safety function of the nuclear power plant in case of the collision with an incoming flying object.
As a correspondence example to guarantee the safety function, it is considered to construct a building by walls and a ceiling with an enough thickness not to generate penetration, scabbing and so on in case that an incoming flying object collides against the building to store a system and an equipment having the safety function. The scabbing means the ejection of irregular pieces of the rear face as a result of the incoming flying object.
An enough thickness of the building construction in case that an incoming flying object collides depends on many conditions. When Chang formula is used which is an evaluation formula relating to a marginal thickness for the scabbing of the rear face caused by an engine collision, a calculation result is obtained that a necessary thickness is 2.0 meters at maximum including an allowance in case of the collision at right angle and in case that concrete strength per unit area is 30 mega pascals. Though there is no possibility that an incoming flying object collides against the roof of the building from the perpendicular direction in reality, it is considered that an enough thickness of the building construction is as many as 1.5 meters in case that an incoming flying object collides.
As an uppermost floor of a boiling for a boiling water reactor is an operating floor that is a large space, it is impossible to install supports for the roof in the vicinity of the center of the operating floor in view of the structure of a nuclear reactor building.
However in case of such a nuclear power plant building where supports are not installed in the vicinity of the center under the roof, if it is tried to construct the building with walls and a ceiling with an enough thickness not to generate penetration and scabbing of the rear face and so on caused by the collision with an incoming flying object, there was a problem that in a roof with a conventional shape such as a planer shape or a convex three dimensional shape, a deflection is generated by a self-weight of a roof with an increased thickness and a formation property in its structure is lowered.
On the other hand, though a roof of a semispherical shape (dome shape) can be formed easily for its construction in case where the body portion is of a cylindrical shape such as in a pressurized water reactor, there was a problem that in case that the building body portion is of a rectangular shape, a formation property in its structure of the building is lowered after all for the reason of a mismatch at the junction of the building body portion and the roof portion.